Gallant Inspiration, and when is it cast?

[Grenage]

Gallant inspiration looks like a great choice for my Bard, but I'm a little unsure when I would cast.

If Bob fails his attack, and I'm two further on in the initiative list, do I cast the spell for his competence bonus on my turn - or early?

I also assume that it's just one attack, not all of the attacks on Bob's turn.

[Manly-man teapot]

It's an immediate action, which means you use it on someone else's turn, and it consumes your swift action on your next turn.

It provides a bonus on one roll only.

[Grenage]

My God, just a swift action; I could cast another spell or attack on my turn? That's pretty amazing!